Hepatocellular carcinoma (HCC) is the 3rd most lethal cancer worldwide. Liver transplant remains the only therapy with a favorable prognosis, and even in these cases, 5-year survival rates are low and recurrence is common 1. Moreover, donor livers are in short supply, and the cost and technical difficulty of transplantation puts therapy out of reach of all by a small minority of people stricken with the disease. Clearly there is a need for pharmaceutical-based therapies for this disease, even if just to extend longevity after transplant. However, the complexities of liver metabolism have kept this goal out of reach. Cytoplasmic thioredoxin reductase (Txnrd1) plays a major role in liver metabolism. Many anti-cancer drugs or drug-candidates, as well as some compounds thought to be active in prophylaxis against cancer initiation, either affect Txnrd1 activity or are substrates for metabolism by Txnrd1 5,6. This suggests that Txnrd1 plays crucial roles in cancer initiation, persistence, and progression. However, prior to now, no studies have been performed in systems having complete and specific genetic disruption of Txnrd1. Therefore, it is unclear what role Txnrd1 plays in responses to these compounds, and what roles undefined 'other targets'might play. We have developed the first animal model with Txnrd1-deficient hepatocytes 3. In the proposed study, we will develop the first animal model with Txnrd1-deficient HCC and to use this model to study initiation, persistence, and progression of HCC in cells that either have or lack Txnrd1. To do this, we have put forth two Specific Aims. First, we will determine whether Txnrd1-deficient hepatocytes are either more susceptible or more refractory than normal hepatocytes to initiation of HCC. Second, we will determine whether Txnrd1 activity is necessary, advantageous, or antagonistic for persistence and progression of HCC cells in situ. This two-year Developmental R21 project is proposed with the intention of establishing and publicly disseminating a novel and powerful animal model for studying HCC, and to provide important understanding of the roles of Txnrd1 in cancer initiation and persistence/progression.